Kinetic energy

Motion energy, and only motion. The AP Physics 1 CED gives translational kinetic energy as $K = \frac{1}{2}mv^2$ and states that it is a scalar quantity.

Scalar has consequences. Kinetic energy has no direction and no components, so two carts rolling toward each other both contribute positive $K$ and nothing cancels. Momentum, being a vector, does cancel. That one difference is why a head on collision can leave a system with zero total momentum and a great deal of energy still to dispose of. $K$ is never negative either: mass is positive and $v^2$ is positive whichever way the object travels.

The square is the part that catches people out. Double the speed and the kinetic energy quadruples; triple it and it goes up ninefold, while doubling the mass only doubles $K$. That asymmetry is behind braking distance. Stopping from twice the speed means the same friction force has to remove four times the energy, so it needs four times the distance.

One subtlety the CED flags: different observers may measure different values of the translational kinetic energy of an object, depending on the observer's frame of reference. A cup of coffee on a moving train has zero kinetic energy according to the passenger and plenty according to someone on the platform.

Rotating bodies carry a second, separate term, $K = \frac{1}{2}I\omega^2$, which adds to the translational one rather than replacing it.